1. Field of the Invention
The present invention relates to a failure diagnostic system that determines whether an evaporated fuel processing system has failed or not in order to prevent evaporated fuel in a fuel tank from being emitted into the air.
2. Description of Related Art
Japanese Laid-Open Patent Publication (Kokai) No. 2000-282972 discloses a failure diagnostic system that determines whether an evaporated fuel processing system has failed or not. This failure diagnostic system is comprised of a first failure diagnostic device (mode C) that determines whether there is any leakage from a large hole with a diameter of about 0.5 cm in a predetermined range based on the engine speed and the engine load as parameters, and a second failure diagnostic system (mode B) that determines that there is leakage from a small hole with a diameter of about 0.02 inch on condition that there is a small change in throttle angle in the predetermined range.
The conventional failure diagnostic system detects a failure caused by leakage from a large hole on condition that the internal pressure of the fuel tank is not reduced to a desired negative pressure. More specifically, the conventional failure diagnostic system determines that the internal pressure of the fuel tank is not reduced to a desired negative pressure, i.e. there is a failure caused by leakage from a large hole on condition that the internal pressure of a tank does not become lower than a predetermined value within a predetermined period of time. In this method, the intake negative pressure that enables a predetermined pressure reduction within a predetermined period of time is normally required, and therefore, the predetermined range of the diagnosis as to whether there is any leakage from a large hole is necessarily determined to be an engine operating range that achieves a certain intake negative pressure. In determining whether there is any leakage from a small hole, the degree of increase in pressure in a sealed fuel tank with the internal pressure thereof having been reduced to a predetermined negative pressure is detected to carry out failure diagnosis, unlike diagnosis as to whether there is any leakage from a large hole as described above. However, the range of the diagnosis as to whether there is any leakage from a small hole is identical with the predetermined range of the diagnosis as to whether there is any leakage from a large hole.
Whether there is any leakage from a small hole is determined according to the degree of increase in the pressure after the pressure reduction, the range of the diagnosis should not necessarily be a range that achieves an intake negative pressure that enables the predetermined pressure reduction in the predetermined period of time. Whether there is any leakage from a small hole may be determined in a longer period of time than the predetermined period of time insofar as the predetermined pressure reduction can be achieved. In the conventional failure diagnostic system, however, this is not taken into consideration in setting the ranges of the diagnosis. Specifically, the range of the diagnosis as to whether there is any leakage from a small hole is only set to be identical with the range of the diagnosis as to whether there is any leakage from a large hole. This unnecessarily limits the range of the diagnosis as to whether there is any leakage from a small hole, and therefore reduces failure diagnosis opportunities.
Further, the conventional failure diagnostic system sets another diagnostic device (mode A) that determines whether there is any leakage from a small hole in air-fuel ratio leaning control during idling with an engine speed being equal to or higher than a predetermined value. This only increases the opportunities for detecting leakage from a small hole by addition of another device and makes the control logic and the like complicated, but cannot efficiently increase failure diagnosis opportunities.
It is therefore an object of the present invention to provide a failure diagnostic system that determines whether an evaporated fuel processing system has failed or not, and that enables an improvement in the failure diagnostic performance by increasing failure diagnosis opportunities without any trouble.
To attain the above object, the present invention provides a failure diagnostic system of an evaporated fuel processing system, comprising: a first failure diagnostic device for shutting off an evaporated fuel purge passage connecting a fuel tank and an engine intake passage from air and determining whether there is any leakage from a large hole by monitoring a degree of increase in pressure in the fuel tank with internal pressure thereof having been reduced to a negative pressure; and a second failure diagnostic device for a pressure in the fuel tank to a predetermined negative pressure and sealing off the fuel tank from air to carry out failure diagnosis to determine whether there is any leakage from a small hole by monitoring a degree of increase in pressure in the fuel tank; and wherein an operating range of the second failure diagnostic device is set to substantially include an operating range of the first failure diagnostic device and to be extended from the operating range of the first failure diagnostic device to include a lower intake negative pressure range.
According to the present invention, the first failure diagnostic device determines whether there is any leakage from a large hole shuts off the purge passage for evaporated fuel, which connects the fuel tank to the engine intake passage, from the air and introduces in the engine intake negative pressure into the fuel tank, and detects poor introduction of the engine intake negative pressure by monitoring the degree of decrease in the internal pressure of the fuel tank. Therefore, the operating range of the first failure diagnostic device is necessarily determined according to the engine intake negative pressure. The second failure diagnostic device that determines whether there is any leakage from a small hole reduces the internal pressure of the fuel tank to the predetermined negative pressure and seals off the fuel tank from air to carry out failure diagnosis to determine whether there is any leakage from a small hole by monitoring the degree of increase in the internal pressure of the fuel tank. Therefore, even if a change in the pressure is small when the negative pressure is introduced, the second failure diagnostic device is capable of failure diagnosis insofar as the pressure can be reduced to the predetermined negative pressure. Therefore, the present invention in which the operating range of the second failure diagnostic device is set to substantially include an operating range of the first failure diagnostic device and to be extended from the operating range of the first failure diagnostic device to include a lower intake negative pressure range makes use of differences in characteristics between the first failure diagnostic device and the second failure diagnostic device, increases the opportunities for failure diagnosis by the second failure diagnostic device without any trouble, and improves the failure diagnostic performance.
In one preferred form of the present invention, the respective operating ranges of the first failure diagnostic device and the second failure diagnostic device are determined according to the engine speed and the engine load determined as parameters, such that the operating range of the second failure diagnostic device includes a lower engine speed range and a lower engine speed range than the operating range of the first failure diagnostic device. It is therefore possible to simply set the optimum operating ranges of the first and second failure diagnostic devices.
In another preferred form of the present invention, the operating range B includes the entire operating range A, it is not only determined whether there is any leakage from a large hole. Therefore, it is never determined that the evaporated fuel purge system is normally operating even though there is leakage from a small hole, and this ensures the reliability of the failure diagnosis.